Mammals have a double circulatory system, in
which blood is moved through vessels by the regular contraction and relaxation
of cardiac muscles in the wall or the heart.
- Blood enters the atria of the heart, flows through open vales into the ventricles, and is then forced out into the arteries during systole.
- The ventricles have thicker walls than the atria, and the left ventricle has a thicker wall than the right ventricle, to allow them to produce a greater force when the muscles contract, necessary so that they can push the blood further.
- In coronary heart disease, the coronary arteries become blocked, so oxygen is not delivered to the heart muscles and they stop contracting. Smoking, stress and a diet high in saturated fats increase the risk.
- Arteries are thick-walled, elastic vessels that carry pulsing, high-pressure blood away from the heart. They split into capillaries, which are tiny vessels with walls only one cell thick. Capillaries take blood close to every cell to the body, so that the cells are supplied with oxygen and nutrients and have their waste products removed. Capillaries join up to form veins. Veins are thin-walled vessels with valves, which carry low-pressure blood back to the heart.
- Blood contains red cells, white cells and platelets floating in plasma. Plasma transports many different substances in solution. Red cells contain the iron-containing protein haemoglobin, which transports oxygen. White cells fight against bacteria and viruses. Platelets help the blood to clot.
- Fluid leaks out of capillaries to fill the spaces between all the body cells, where it is called tissue fluid. It is collected into lymph vessels which carry it back to the bloodstream.
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